Stepped optical bridge for connecting semiconductor waveguides

ABSTRACT

A photonic device includes a semiconductor wafer having a waveguide formed therein. An end of the waveguide includes a step. The photonic device further includes a semiconductor chip bonded to the semiconductor wafer and having an active region, and a waveguide coupler disposed in a gap between a sidewall of the semiconductor chip and the end of the waveguide. The waveguide coupler includes an optical bridge that has a first end and a second end opposing the first end. The first end of the optical bridge is interfaced with a facet of the active region of the semiconductor chip. The second end of the optical bridge is interfaced with the end of waveguide, and has a portion thereof disposed over the step at the end of the waveguide.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/426,366, filed on Feb. 7, 2017, entitled “Stepped Optical Bridge ForConnecting Semiconductor Waveguides,” which application claims priorityto U.S. Provisional Application No. 62/292,633, filed on Feb. 8, 2016,entitled “High-Speed Optical Transmitter with a Silicon Substrate,” U.S.Provisional Application No. 62/292,675, filed on Feb. 8, 2016, entitled“Stepped Optical Bridge for Connecting Semiconductor Waveguides,” andU.S. Provisional Application No. 62/292,636, filed on Feb. 8, 2016,entitled “Broadband Back Mirror for a III-V Chip in Silicon Photonics,”the disclosures of which are incorporated by reference for all purposes.

The entire disclosures of the following U.S. patent applications areincorporated by reference into this application for all purposes:application Ser. No. 15/426,823, filed on Feb. 7, 2017, entitled“High-Speed Optical Transmitter with a Silicon Substrate;” andapplication Ser. No. 15/426,375, filed on Feb. 7, 2017, entitled“Broadband Back Mirror for a III-V Chip in Silicon Photonics.”

BACKGROUND OF THE INVENTION

Silicon integrated circuits (“ICs”) have dominated the development ofelectronics and many technologies based upon silicon processing havebeen developed over the years. Their continued refinement led tonano-scale feature sizes that can be important for making metal oxidesemiconductor CMOS circuits. On the other hand, silicon is not adirect-bandgap material. Although direct-bandgap materials, includingIII-V semiconductor materials, have been developed, there is a need inthe art for improved methods and systems related to photonic ICsutilizing silicon substrates.

SUMMARY OF THE INVENTION

According to an embodiment of the present invention, a photonic deviceincludes a semiconductor wafer. The semiconductor wafer includes a baselayer, a first insulating layer disposed over a surface of the baselayer, a device layer disposed over the first insulating layer, and awaveguide formed in the device layer. The semiconductor wafer has arecess in the device layer and the first insulating layer. The recessexposes a portion of the surface of the base layer and forms a sidewallexposing an end of the waveguide. The end of the waveguide includes astep having a first length and a first height. The first height is lessthan a thickness of the waveguide. The photonic device further includesa semiconductor chip bonded to the surface of the base layer of thesemiconductor wafer. The semiconductor chip includes an active region. Afacet of the active region is exposed at a sidewall of the semiconductorchip. There is a gap between the sidewall of the semiconductor chip andthe sidewall of the recess. The photonic device further includes awaveguide coupler disposed in the gap. The waveguide coupler includes asecond insulating layer disposed over the surface of the base layer ofthe semiconductor wafer, and an optical bridge disposed over the secondinsulating layer. The optical bridge has a first end and a second endopposing the first end. The first end of the optical bridge isinterfaced with the facet of the active region. The second end of theoptical bridge is interfaced with the end of waveguide, and has aportion thereof disposed over the step at the end of the waveguide.

According to another embodiment of the present invention, a method forfabricating a photonic device includes providing a semiconductor wafer.The semiconductor wafer includes a base layer, a first insulating layerdisposed over a surface of the base layer, a device layer disposed overthe first insulating layer, and a waveguide formed in the device layer.The method further includes performing etching to form a recess in thedevice layer and the first insulating layer. The recess exposes aportion of the surface of the base layer, and has a sidewall exposing anend of the waveguide. The method further includes bonding asemiconductor chip to the exposed portion of the surface of the baselayer. The semiconductor chip includes an active region. A facet of theactive region is exposed at a sidewall of the semiconductor chip. Thereis a gap between the sidewall of the semiconductor chip and the sidewallof the recess. The method further includes forming a second insulatinglayer in the gap, performing etching to remove a portion of thewaveguide to form a step at the end of the waveguide, and forming anoptical bridge over the second insulating layer. The optical bridge hasa first end and a second end opposing the first end. The first end ofthe optical bridge is interfaced with the facet of the active region.The second end of the optical bridge is interfaced with the end ofwaveguide, and has a portion thereof disposed over the step at the endof the waveguide.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic plan view of a portion of a photonic deviceaccording to an embodiment of the present invention.

FIGS. 2-10 show schematic cross-sectional views illustrating the stepsof a method of forming a photonic device according to embodiments of thepresent invention.

FIG. 11 shows a simplified flowchart illustrating a method of forming aphotonic device according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates generally to an integrated photonic deviceand methods of making same. More specifically, the present inventionrelates to an integrated photonic device that includes a semiconductorwafer having a waveguide formed therein, a semiconductor chip bonded tothe semiconductor wafer and having an active region, and a waveguidecoupler for coupling the waveguide and the active region of thesemiconductor chip.

FIG. 1 shows a schematic plan view of a portion of an integratedphotonic device 100 according to an embodiment of the present invention.The photonic device 100 includes a silicon photonic grating 140 and awaveguide 130, which are formed in a device layer of asilicon-on-insulator (SOI) wafer. The photonic device 100 furtherincludes a semiconductor chip 110. In some embodiments, thesemiconductor chip 110 comprises a III-V material and includes aback-side mirror 160 and a laser ridge 120 formed therein. Thesemiconductor chip 110 is bonded to the SOI wafer. A waveguide coupler150 is formed on the SOI wafer to provide optical coupling between thewaveguide 130 and the laser ridge 120.

FIGS. 2-10 show schematic cross-sectional views illustratingintermediate steps of a method of forming an integrated photonic device200 according to embodiments of the present invention.

Referring to FIG. 2, a semiconductor wafer is provided. Thesemiconductor wafer includes a base layer 210, and a buried oxide (BOX)layer 220 disposed over a surface 212 of the base layer 210. In someembodiments, the base layer 210 comprises silicon, and the BOX layer 220comprises silicon oxide. A waveguide 230 is formed in a device layerover the BOX layer 220. The waveguide 230 comprises crystalline siliconin an embodiment. Crystalline silicon has a higher refractive indexcompared to that of silicon oxide. Thus, the BOX layer 220 serves as alower cladding layer for the waveguide. An upper cladding layer 240 isformed over the waveguide 240. The upper cladding layer 240 comprisessilicon oxide in an embodiment. The upper and lower cladding layersprovide traversal (vertical) optical confinement in the waveguide 240.The waveguide 240 may also be surrounded by a cladding material in theplane of the device layer for lateral optical confinement.

Still referring to FIG. 2, a recess 204 is formed on the semiconductorwafer. Walls in the semiconductor wafer define the recess 204. Therecess 204 runs through the device layer and the BOX layer 220 to exposea portion of the surface 212 of the base layer 210. The recess 204 has asidewall 222 that exposes an end of the waveguide 230. The sidewall 222of the recess 204 is one of the walls defining the recess. The recess204 may be formed by etching, using techniques such as dry etch, wetetch, chemically assisted ion beam etch, reactive ion etch, or the like.One of ordinary skill in the art would recognize many variations,alternatives, and modifications.

Referring to FIG. 3, a semiconductor chip 250 is bonded to the exposedportion of the surface 212 of the base layer 210 via a bonding structure214. U.S. patent application Ser. No. 14/509,971 describes in moredetail the process of bonding a semiconductor chip to a semiconductorwafer, the content of which is incorporated herein by reference in itsentirety. There is a gap 206 between the sidewall 222 of the recess 204and a sidewall 258 of the semiconductor chip 250. The size of the gap206 may range from a few microns (e.g., 5 μm) to greater distances(e.g.,200 μm) according to various embodiments. In some embodiments,semiconductor chip 250 includes a substrate 251 that comprises InP,GaAs, InGaAs, InGaAsP, AlGaInAs, GaN, or the like. The semiconductorchip 250 also includes an active region 252 that comprises opticallyactive materials that can produce optical gain, variable optical phase,optical attenuation, absorption, or the like. In some embodiments, theactive region 252 comprises a multiple quantum well (MQW) structure. Theactive region 252 is aligned with the waveguide 230 along an opticalpath. An end facet 253 of the active region 252 is exposed at thesidewall 258 of the semiconductor chip 250.

In the following, the steps of forming a waveguide coupler between thewaveguide 230 and the active region 252 is described with reference toFIGS. 4-10.

Referring to FIG. 4, an insulating layer 260 is formed in the gap 206between the sidewall 222 of the recess 204 and the sidewall 258 of thesemiconductor chip 250. The upper surface 262 of the insulating layer260 is above the upper surface 254 of the active region 252 and theupper surface 236 of the waveguide 230, and below the upper surface 256of the semiconductor chip 250 by a height difference 270. U.S. Pat. No.9,097,846 describes a process of forming an insulating layer in the gap,the content of which is incorporated herein by reference in itsentirety. In some embodiments, a portion of the upper cladding layer 240located above an end of the waveguide 230-1 is removed, such that theremaining portion of the upper cladding layer 240-1 located above theend of the waveguide 230-1 is thinner than the rest of the uppercladding layer 240. According to various embodiments, the upper surfaceof the remaining portion of the upper cladding layer 240-1 located abovethe end of the waveguide 230-1 may or may not be flush with the uppersurface 262 of the insulating layer 260.

The insulating layer 260 comprises silicon oxide in some embodiments.The insulating layer 260 may be formed by deposition (e.g., usingtechniques such as chemical vapor deposition (CVD), plasma enhanced CVD(PECVD), sputtering, sub-atmospheric CVD (SACVD), physical vapordeposition (PVD), evaporation, atomic layer deposition (ALD), a spin-onprocess, or the like). The insulating layer 260 may also be depositedabove the end portion of the waveguide 230-1. In some embodiments, theinsulating layer 260 may comprise other dielectric materials, such asSi₃N₄, Si_(x)O_(y), Si_(x)O_(y)N_(z), Si_(x)N_(y), or the like.Depending on the mode profile in the waveguide 230 and the active region252, the refractive index of the insulating layer 260 may be selected toprovide a good cladding performance for a high coupling efficiencybetween the waveguide 230 and the active region 252 during the opticalbridge formation process to be described below.

Referring to FIG. 5, a spacer layer 280 is formed (e.g., by depositionusing CVD, PECVD, SACVD, PVD, evaporation, ALD, a spin-on process, orthe like). The deposition of the spacer layer 280 is conformal so thatthe spacer layer 280 conforms to a topography of the surface of thephotonic device, covering the upper surface of the upper cladding layer240 (including the upper surface of the remaining portion of the uppercladding layer 240-1 located above the end of the waveguide 230-1), asidewall of the upper cladding layer 240, the upper surface 262 of theinsulating layer 260, the sidewall 258 of the semiconductor chip 250,and the upper surface 256 of the semiconductor chip 250. The materialfor the spacer layer 280 may be selected from silicon oxide, siliconnitride, aluminum oxide, hafnium oxide, or other suitable dielectricmaterial. According to some embodiments, the spacer layer 280 comprisessilicon oxide.

Still referring to FIG. 5, the spacer layer 280 includes a first section280-1 and a second section 280-2. The first section of the spacer layer280-1 is over a first section of the insulating layer 260-1 that abutsthe sidewall 222 of the recess 206. The second section of the spacerlayer 280-2 is over the second section of the insulating layer 260-2that abuts the sidewall 258 of the semiconductor chip 250. The secondsection of the spacer layer 280-2 conforms to the sidewall 258 of thesemiconductor chip 250, and has a height 284 that is approximately equalto the thickness 282 of the spacer layer 280 plus the height difference270 between the upper surface 256 of the semiconductor chip 250 and theupper surface 262 of the insulating layer 260. Because the top portionof the sidewall 258 of the semiconductor chip 250 is exposed before thespacer layer 280 is formed, the second section of the spacer layer 280-2is self-aligned against the sidewall 258 of the semiconductor chip 250.In some embodiments, the thickness 282 of the spacer layer 280 may rangefrom 0.01 μm to 1 μm, and the height 284 of the second section of thespacer layer 280-2 may range from 0.2 μm to 4 μm. In some embodiments,the height 284 of the second section of the spacer layer 280-2 istypically much greater than the thickness 282 of the first section ofthe spacer layer 280-1 (e.g., the height 284 of the second section ofthe spacer layer 280-2 is more than double or triple the thickness 282of the first section of the spacer layer 280-1).

Referring to FIG. 6, a photoresist layer 288 is formed. A window in thephotoresist layer 288 is created by using a mask to expose the area inthe gap 206 between the sidewall 258 of the semiconductor chip 250 andthe sidewall 222 of the recess 204. The window is sufficiently large sothat the second section of the spacer layer 280-2 is exposed, as well asa portion of the spacer layer 280 above the end portion of the waveguide230-1.

Referring to FIG. 7, a dry etch process step is performed. The etchingremoves the first section of the spacer layer 280-1. A portion of thefirst section of the insulating layer 260-1 is also removed, such thatthe upper surface 264 of the remaining first portion of the insulatinglayer 260-1 is slightly below, slightly above, or flush with the lowersurface 238 of the waveguide 230, to facilitate the formation of anoptical bridge to be described below. Because dry etching can beoptimized to be a vertical process (i.e. anisotropic), and because theheight 284 of the second section of the spacer layer 280-2 is muchgreater than the thickness 282 of the first section of the spacer layer280-1, in some embodiments, only a portion of the second section of thespacer layer 280-2 is removed and the second section of the insulatinglayer 260-2 is not removed. In addition, the etching also removes aportion of the spacer layer 280 disposed on the upper cladding layer240, and a portion of the upper cladding layer 240. In some embodiments,the spacer layer 280 comprises a different material than that of theinsulating layer 260, and has a lower etch rate than that of theinsulating layer 260. In that case, the difference between the height284 of the second section of the spacer layer 280-2 and the thickness282 of the first section of the spacer layer 280-1 may not need to be aslarge. According to another embodiment, the spacer layer 280 comprisessilicon oxide having a density higher than that of the silicon oxide inthe second insulating layer 260, and thus has a lower etch rate that ofthe second insulating layer 260.

The second section of the insulating layer 260-2 serves as aself-aligned protective layer that covers the facet 253 of the activeregion 252, and therefore prevents damages to the active region 252 frommechanical and chemical attack during the etch and during subsequentprocesses such as photoresist stripping and wafer cleaning, and thelike. It is understood that, even if all of the second section of thespacer layer 280-2 and a portion of the second section of the insulatinglayer 260-2 are etched away, as long as the remaining portion of thesecond section of the insulating layer 260-2 is higher than the uppersurface 254 of the active region 252, the active region 252 would beprotected. By using the spacer layer 280 to create a self-aligned thinprotective layer in front of the active region 252, it is not necessaryto align the window of the photoresist layer 288 precisely. Therefore,in this method, as long as the window of the photoresist layer 288 isdimensioned such that an edge 287 of the window is located somewhereabove the semiconductor chip 250 so that the second section of thespacer layer 280-2 is exposed, the etching process will create the thinprotective layer in front of the facet 253 of the active region 252.

Still referring to FIG. 7, in the etching process, a portion of the endof the waveguide 230-1 is also removed. Because the etch process can beoptimized in such a way that the etch-rate of silicon oxide in thesecond insulating layer 260 is greater than that of crystalline siliconin the waveguide 230, not the entire end of the waveguide 230-1 isremoved. Thus, the remaining portion of the end of the waveguide 230-1forms a step having a height 232 and a length 234. The step height 232may be controlled by selecting an appropriate etch ratio between the twomaterials. For example, an etch ratio of 1:20, 1:10, 1:3, 1:2, or thelike, may be selected. According to some embodiments, an etch ratio ofapproximately 1:2 is selected, such that the height 232 of the step230-1 is approximately one half of the height of the waveguide 230.

Referring to FIG. 8, the photoresist layer 280 is removed.

Referring to FIG. 9, a portion of the second section of the spacer layer260-2 is removed so that the upper surface of the remaining portion ofthe second section of the insulating layer 260-2 is flush with the uppersurface of the remaining portion of the first section of the insulatinglayer 260-1. The facet 253 of the active region 252 is exposed for theformation of an optical bridge as described below.

Referring to FIG. 10, an optical bridge 290 is formed over the secondinsulating layer 260. U.S. Pat. No. 9,097,846 describes a process offorming an optical bridge, the content of which is incorporated hereinby reference in its entirety. The optical bridge 290 provides opticalcoupling between the waveguide 230 and the active region 252 of thesemiconductor chip 250. The optical bridge 290 comprises a material witha refractive index higher than that of silicon oxide in the insulatinglayer 260 to provide optical confinement in the optical bridge 290.According to some embodiments, the optical bridge 290 comprisesamorphous silicon (a-Si). The optical bridge 290 may be formed bydeposition using any appropriate deposition technique.

Still referring to FIG. 10, the first end of the optical bridge 290-2mates with the active region 252 at the sidewall 258 of thesemiconductor chip 250. A portion of the second end of the opticalbridge 290-1 is disposed on the step at the end of the waveguide 230-1,such that the optical bridge 290 has an L-shaped cross-section at theinterface. According to some embodiments, the height of the step 230-1ranges from about 5% to about 95% of the thickness of the waveguide 230.In some other embodiments, the height of the step 230-1 ranges fromabout 10% to about 85%, or from about 20% to about 75%, or from about30% to about 65%, or from about 40% to about 55%, of the thickness ofthe waveguide 230. The length of the interface region may be selected byselecting an appropriate length 234 of the step 230-1. In someembodiments, the length of the interface region ranges from about 0.1 μmto about 50 μm.

In some other embodiments, the length of the interface region rangesfrom about 0.5 μm to about 50 μm, or from about 1 μm to about 50 μm, orfrom about 5 μm to about 50 μm. In one embodiment, the height of thestep 230-1 is approximately one-half of the thickness of the waveguide230. Thus, the effective refractive index in the interface region isapproximately an average of the refractive index of the optical bridgeand that of the waveguide 230. In other embodiments, the ratio of theheight of the step 230-1 to the thickness of the waveguide 230, as wellas the length 234 of the step 230-1, may be designed to have othervalues as needed to optimize the effective refractive index of theinterface region to enable optimized optical coupling between thewaveguide 230 and the active region 252.

According to an embodiment of the present invention, a method of forminga photonic device includes providing a semiconductor wafer 202. Thesemiconductor wafer includes a base layer 210 and a first insulatinglayer 220. The method further includes forming a waveguide 230 over thefirst insulating layer 220; performing etching to form a recess 204 inthe semiconductor wafer 202; and bonding a semiconductor chip 250 to thesemiconductor wafer 202 in the recess 204. The semiconductor chip 250includes an active region 252. The method further includes forming asecond insulating layer 260 in a gap 206 between a sidewall 258 of thesemiconductor chip 250 and a sidewall 222 of the recess 204; and forminga spacer layer 280. The spacer layer 280 includes a first section 280-1disposed over a first section 260-1 of the second insulating layer 260that abuts the sidewall 222 of the recess 204, and a second section280-2 disposed over a second section 260-2 of the insulating layer 260that abuts the sidewall 258 of the semiconductor chip 250. The methodfurther includes performing an etch to remove the first section 280-1 ofthe spacer layer 280 and a portion of the first section 260-1 of theinsulating layer 260 to expose the end of the waveguide 230 withoutexposing the active region 252 of the semiconductor chip 250.

In some embodiments, the base layer of the semiconductor wafer mayinclude silicon. The first insulating layer and the second insulatinglayer may include silicon oxide. The waveguide may include crystallinesilicon. The semiconductor chip may include a III-V material. The activeregion may include a multiple-quantum-well (MQW) structure. The spacerlayer may include a dielectric material. In some embodiments, thedielectric material is selected from one of silicon oxide, siliconnitride, aluminum oxide, hafnium oxide, or a combination thereof. Insome embodiments, the spacer layer is formed by chemical vapordeposition (CVD), plasma enhanced CVD (PECVD), sputtering,sub-atmospheric CVD (SACVD), physical vapor deposition (PVD),evaporation, or atomic layer deposition (ALD). In an embodiment, thesecond height of the second section of the spacer layer is substantiallyequal to a sum of the first height and the thickness of the firstsection of the spacer layer.

In some embodiments, the method may further include, forming a thirdinsulating layer over the waveguide before forming the spacer layer. Aportion of the spacer layer is disposed over the third insulating layer.In some embodiments, the base layer of the semiconductor wafer includessilicon; the waveguide includes crystalline silicon; and the firstinsulating layer, the second insulating layer, and the third insulatinglayer includes silicon oxide.

In some embodiments, the method further includes removing a portion ofthe second section of the second insulating layer to expose the activeregion after the first section of the spacer layer and the portion ofthe first section of the second insulating layer are removed. Theportion of the second section of the second insulating layer is removedby one of dry etch, wet etch, chemically assisted ion beam etch, orreactive ion etch.

In some embodiments, the spacer layer is a thin film depositedconformally on the semiconductor wafer. The spacer layer may provide aproperty for selecting etch with respect to the second insulating layer.The spacer layer may also provide a property for ease of removal.

FIG. 11 shows a simplified flowchart illustrating a method of forming aphotonic device according to another embodiment of the presentinvention. The method includes, at 1102, providing a semiconductor wafer202. The semiconductor wafer includes a base layer 210 and a firstinsulating layer 220. The method further includes, at 1104, forming awaveguide 230 over the first insulating layer 220; at 1106, performingetching to form a recess 204 in the semiconductor wafer 202; and at1108, bonding a semiconductor chip 250 to the semiconductor wafer 202 inthe recess 204. The semiconductor chip 250 includes an active region252. The method further includes, at 1110, forming a second insulatinglayer 260 in a gap 206 between a sidewall 258 of the semiconductor chip250 and a sidewall 222 of the recess 204; and at 1112, performingetching to remove a portion of the second insulating layer 260 to exposean end of the waveguide 230-1, and to remove a portion of the end of thewaveguide 230-1, thereby forming a step 230-1 at the end of thewaveguide 230. The method further includes, at 1114, forming an opticalbridge 290 over the second insulating layer 260. A first end of theoptical bridge 290 is interfaced with the active region 252 of thesemiconductor chip 250. A second end of the optical bridge 290 has aportion 290-1 that is disposed over the step 230-1 at the end of thewaveguide 230.

It should be appreciated that the specific steps illustrated in FIG. 11provide a particular method of forming a photonic device according to anembodiment of the present invention. Other sequences of steps may alsobe performed according to alternative embodiments. For example,alternative embodiments of the present invention may perform the stepsoutlined above in a different order. Moreover, the individual stepsillustrated in FIG. 11 may include multiple sub-steps that may beperformed in various sequences as appropriate to the individual step.Furthermore, additional steps may be added and some steps may be removeddepending on the particular applications. One of ordinary skill in theart would recognize many variations, modifications, and alternatives.

The specific details of particular embodiments may be combined in anysuitable manner without departing from the spirit and scope ofembodiments of the invention. However, other embodiments of theinvention may be directed to specific embodiments relating to eachindividual aspect, or specific combinations of these individual aspects.

The above description of exemplary embodiments of the invention has beenpresented for the purposes of illustration and description. It is notintended to be exhaustive or to limit the invention to the precise formdescribed, and many modifications and variations are possible in lightof the teaching above. The embodiments were chosen and described inorder to explain the principles of the invention and its practicalapplications to thereby enable others skilled in the art to utilize theinvention in various embodiments and with various modifications as aresuited to the particular use contemplated.

1. (canceled)
 2. A device comprising: a semiconductor waveguide havingan end, wherein: the end of the semiconductor waveguide comprises afirst step having a first length and a first height; and the firstheight is less than a maximum thickness of the semiconductor waveguide;an optical device; and a waveguide coupler forming an optical bridgebetween the semiconductor waveguide and the optical device, wherein; theoptical bridge has a first end opposite a second end; the second end ofthe optical bridge has a second step; the second step has a secondlength and a second height; the second height is less than a maximumthickness of the optical bridge; and the second step of the waveguidecoupler is disposed over the first step of the semiconductor waveguide.3. The device of claim 2, wherein: the optical device is a semiconductorchip with an active region for generating light; and the optical bridgeoptically connects the active region with the semiconductor waveguide.4. The device of claim 2, wherein: the semiconductor waveguide is in adevice layer of a semiconductor device having walls forming a pit in thesemiconductor device; and the optical device is bonded in the pit of thesemiconductor device.
 5. The device of claim 2, wherein thesemiconductor waveguide, the optical device, and the waveguide couplerare integrated on a substrate.
 6. The device of claim 2, wherein: thesemiconductor waveguide is made of crystalline silicon; and the opticaldevice is made of III-V material.
 7. The device of claim 2, wherein theoptical bridge comprises amorphous silicon.
 8. The device of claim 2,wherein the waveguide coupler comprises a higher refractive indexmaterial next to the optical bridge to confine light in the opticalbridge.
 9. The device of claim 2, wherein the first height of the firststep ranges from 30% to 65% of the maximum thickness of thesemiconductor waveguide.
 10. The device of claim 2, wherein the firstlength of the first step is equal to or greater than 5 microns and equalto or less than 50 microns.
 11. The device of claim 2, wherein thesecond length is equal to the first length.
 12. The device of claim 2,wherein the optical device is a gain medium for a laser.
 13. A methodfor fabricating a device, the method comprising: forming a semiconductorwaveguide in a semiconductor wafer, wherein: the semiconductor wafercomprises a substrate; the semiconductor waveguide is integrated on thesubstrate; the semiconductor waveguide comprises a first step having afirst length and a first height; and the first height is less than amaximum thickness of the semiconductor waveguide; and forming awaveguide coupler on the substrate to optically couple light from thesemiconductor waveguide with an optical device integrated on thesubstrate, wherein: the waveguide coupler comprises an optical bridgeextending from the semiconductor waveguide to the optical device; theoptical bridge has a first end opposite a second end; the second end ofthe optical bridge has a second step; the second step has a secondlength and a second height; the second height is less than a maximumthickness of the optical bridge; and the second step of the waveguidecoupler is disposed over the first step of the semiconductor waveguide.14. The method of claim 13, wherein the first length is equal to thesecond length.
 15. The method of claim 13, further comprising: forming apit in the semiconductor wafer; and bonding the optical device in thepit of the semiconductor wafer before forming the waveguide coupler. 16.The method of claim 13, wherein forming the waveguide coupler comprises:etching a portion of the semiconductor waveguide to form the first step;and depositing material to form the optical bridge and the second stepof the optical bridge over the first step of the semiconductorwaveguide.
 17. The method of claim 13, further comprising covering theoptical device and the waveguide coupler to hermetically seal theoptical device.
 18. A method for using a device, the method comprising:transmitting light from an optical device to a waveguide coupler; andguiding light in the waveguide coupler to a waveguide, wherein thewaveguide comprises a first step having a first length and a firstheight; the first height is less than a maximum thickness of thewaveguide; the waveguide coupler comprises an optical bridge extendingfrom the optical device to the waveguide; the optical bridge has a firstend opposite a second end; the second end of the optical bridge has asecond step; the second step has a second length and a second height;the second height is less than a maximum thickness of the opticalbridge; and the second step of the waveguide coupler is disposed overthe first step of the waveguide.
 19. The method of claim 18, furthercomprising generating light using the optical device.
 20. The method ofclaim 18, wherein: the optical device, the waveguide coupler, and thewaveguide are integrated on a substrate; the waveguide comprisescrystalline silicon; and the optical bridge is made of amorphoussilicon.
 21. The method of claim 18, wherein the first length is equalto the second length.